This invention relates to the art of papermaking and more particularly to an apparatus and method of treating paper pulp stock. Paper pulp of relatively low consistency (3 to 8% fiber content) has been fed to a refiner, such as a disc refiner. There, the individual fibers of the pulp stock slurry are cut and/or fractured so as to render the stock suitable for papermaking.
For the purpose of increasing the efficiency of such papermaking operations, it is desirable to treat pulp by a disc refiner having a higher fiber content, such as a fiber content of from 8 to 18%. However, such attempts have usually failed because this higher fiber content results in a jamming or clogging of the disc refiner. Once the refiner commences to clog, total clogging occurs shortly thereafter with the result that the disc refiner is no longer operable. In attempting to feed medium pulp consistencies to a disc refiner, such as pulp stock having a fiber content of from 8 to 18%, flocculation of the fibers with the slurry causes the above described clogging of the disc refiner.
Prior to this invention, mechanical devices for fluidizing medium consistency paper stock had been known. Such devices include a fluidizing device sold by Kamyr, Inc. under the brand name MC Pump, and is characterized by the seller as a centrifugal, thickstock pump. This is a rotary device for deflocculating paper pulp. Such devices have previously been useful, for example, in pumps for pumping paper stock of medium consistency from a storage tank to another portion of the papermaking system. It has been found that medium consistency pulp exhibits high pipe-clogging characteristics and a fluidizer, such as the Kaymr MC Pump, serves to deflocculate the medium consistency pulp in a pulp storage tank or chest, to thereby enable it to be passed in hydraulic conduits without clogging.
Another common configuration for a pulp refiner is a rotating flat disc attached at a right angle to an end of a shaft. In comparison with the disc refiners, conical refiners, such as the subject of the present invention, have been found to be advantageous with respect to refined pulp properties and energy consumption. Comparison trials made during the past two years showed that paper made from pulp refined conically has 5 to 15% better strength properties than that made from disc refined pulp. At the same time, energy required to develop a given level of paper strength was 30 to 40% less for the conical refiner.
More uniform fiber treatment by the conical refiner is an apparent explanation for its superior results. Because the speed of the moving bars in a refiner determines the quality of treatment of the fibers and the amount of energy required to shear the pulp slurry, an ideal refiner would have all positions along the bars moving at equal speeds. This suggests a cylindrical refiner, but a cylindrical refiner which allows a pulp slurry to be passed through it continuously in an axial direction has been found to be almost impossible to design feasibly. The closest and most successful approach to this ideal configuration is the conical refiner, the subject of this invention.
Because a disc refiner's configuration resembles that of a pump, a considerable portion of the energy it consumes is expended in pumping the pulp slurry rather than refining it. Depending on its speed of rotation, from 50 to 75% of the power required by a disc refiner can go into pumping rather than refining. Considerably less pumping, and therefore considerably less secondary function power usage, is done by a conical refiner. The conical refiner's configuration also lends itself better to the insertion of a pulp fluidizer at its inlet. As will be discussed in detail in the description of an embodiment of the present invention, the flow of pulp through the fluidizer and the refiner follows a straight path and the distance between the two sections can be minimized. In a nominal sized conical refiner, a distance of one inch produces a transient time of only 0.07 seconds at a refining flow rate of 200 gallons per minute. Due to this close proximity of the conical refiner to the fluidizer, a shorter dwell time can be achieved than in the case of a disc configuration, and it is the shorter dwell time which inhibits reflocculation of the pulp, which is the object of the device of the present invention.